An electrode for a new type of secondary cell (or battery) has been intensely studied in order to enhance the energy density and out-put of the cell. For example, it has been proposed to employ, as the active material for the positive electrode, a transition metal oxide or a chalcogenide compound (such as TiS.sub.2, MoS.sub.2, CoO.sub.2, V.sub.2 O.sub.5, FeS.sub.2, NbS.sub.2, ZrS.sub.2, NiPS.sub.3, VSe.sub.2, MnO.sub.2 and the like), a graphite compound prepared by heat-polymerizing organic materials, carbon fluoride, graphite, or an electroconductive polymer material (such as polyacetylene, polypyrrole, polyaniline, polyazulene and the like). As the active material for the negative electrode, a composite electrode using lithium, lithium alloy and graphite has also been proposed. The proposed electrode active materials are capable of lightening the weight and enhancing the energy density of the cell in comparison with conventional materials.
However, the proposed electrode materials have some problems to be solved in order to be actually mounted in such cells. Especially, one important problem resides in the current collective ability of the cell and in the formation of an electrode from the active material and therefore it is desired to develop a composite sheet electrode which is capable of fully collecting the electric current from the electrode materials.
Conventional pellets for coin type cells, for example, are prepared by mixing carbon powder, an electrode active material and optionally a bonding material (such as Teflon.RTM. powder) and then pressure-molding. The carbon powder in these pellets acts as a current collecting material, but if the amount of carbon powder is increased in order to enhance the current collecting ability, the other properties of the cell such as energy density and mechanical strength are deteriorated. Thus, the amount of carbon powder is limited to a certain degree and its discharge capacity is also limited. Also, it is difficult to form sheet electrodes from the conventional pellets.
In order to resolve the above problems, British Patent Specification 1,216,549 proposes the electroconductive polymer material listed above, which can be formed into a sheet which has good current collecting ability. The patent states that the proposed polymer material enhances the energy density of a polymer secondary cell. The electrode employing the proposed polymer material, however, still has poor current collecting ability in positive electrodes and the performance of the electroconductive polymer material is poor.
In order to resolve the above problems, Japanese Kokai Publication 58-206066 proposes an electrode which comprises a gelled material of polyacetylene containing toluene, a porous metal material and a carbon fiber. Japanese Kokai Publication 61-206066 also proposes an electrode which comprises a film of fibrous polyaniline obtained by an electropolymerization process in which carbon particles are dispersed. These electrodes, however, have a poor current collecting ability and poor strength.
Japanese Kokai Publications 62-176046 and 63-259965 disclose pellets which are prepared by filling the electroconductive polymer material in the pores of a porous substrate. However, since the pores are not completely continuous, it is difficult to fully fill the polymer material in them. Even if the pores are fully filled, the electrode reaction does not smoothly progress because of some incontinuous pores.
Japanese Kokai Publication 1-300814 further proposes a composite electrode which is prepared by filling the active material as an electrode in the pores of a continuous porous sheet of carbon which has good current collecting ability. However, in order to fill the electrode-active material, the porosity of the porous sheet must be increased, and the higher the porosity, the lower the strength.